Typical control systems include actuators, which exert forces in various directions and generate rotational forces or moments about the center of mass of the vehicle, and thus rotate the vehicle in pitch, roll, or yaw. For example, a pitching moment is a vertical force applied at a distance forward or aft from the center of mass of the vehicle, causing the vehicle to pitch up or down. Roll, pitch and yaw refer, in this context, to rotations about the respective axes starting from a defined equilibrium state. The equilibrium roll angle is known as the wing level or zero bank angle for aircraft, equivalent to a level heeling angle on a ship. Yaw is known as “heading”. The equilibrium pitch angle in submarine and airship parlance is known as “trim”, but in aircraft this usually refers to angle of attack, rather than orientation. However, common usage ignores this distinction between equilibrium and dynamic cases.
A fixed wing aircraft increases or decreases the lift generated by the wings when it pitches nose up or down by increasing or decreasing the angle of attack. The roll angle is also known as bank angle on a fixed wing aircraft, which usually “banks” to change the horizontal direction of flight. An aircraft is usually streamlined from nose to tail to reduce drag, making it typically advantageous to keep the sideslip angle near zero.
The forces acting on a spacecraft are of three types: propulsive force (usually provided by the vehicle's engine thrust); gravitational force exerted by the Earth and other celestial bodies; and aerodynamic lift and drag (when flying in the atmosphere of the Earth or other body). The vehicle's attitude must be taken into account because of its effect on the aerodynamic and propulsive forces. There are other reasons, unrelated to flight dynamics, for controlling the vehicle's attitude in non-powered flight (e.g., thermal control, solar power generation, communications, or astronomical observation). The flight dynamics of spacecraft differ from those of aircraft in that the aerodynamic forces are of very small or vanishingly small effect for most of the vehicle's flight, and cannot be used for attitude control during that time. Also, most of a spacecraft's flight time is usually unpowered, leaving gravity as the dominant force.
Thus, orientation dynamics are critical for the performance of vehicles operating in all domains. With the emergence of unmanned, autonomous, miniature, micro-sized or nano-sized vehicles, conventional control surfaces cannot always be used due to size, weight and power restrictions. Therefore, there is a need for a new low-power, low-profile, compact control system for these types of vehicles.